1. Field of the Invention
This invention relates to a method and apparatus for maintaining quality control in a laser peening apparatus, and more particularly, to a method and apparatus using test elements that will deflect when subjected to a laser pulse for ascertaining the magnitude of the impulse associated therewith.
2. Description of the Related Art
Laser shock processing, or laser shock peening, or laser peening, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser pulses directed onto the surface area of a workpiece. Laser shock processing is an effective method of increasing fatigue resistance in metals by treating fatigue critical regions. For a more thorough background in the prior history of laser shock processing, a reference can be made to U.S. Pat. Nos. 5,131,957 and 5,741,559, such patents are explicitly hereby incorporated by reference.
Laser shock processing, as understood in the art and used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted, coated, or un-coated surface. Laser peening has been utilized to create a compressively stressed layer in the subsurface of a workpiece, thereby considerably increasing the resistance of the workpiece to fatigue failure. Laser shock processing typically utilizes two overlays: a transparent overlay (usually water) and an opaque overlay, typically an oil-based, acrylic-based, or water-based paint or tape. Laser shock processing can also utilize only a transparent overlay on a bare surface. During processing, a laser beam is directed to pass through the transparent overlay and is absorbed by the opaque overlay or bare surface, causing vaporization of a portion of the opaque overlay or bare surface, which results in rapid plasma formation and the generation of a high amplitude shock wave. The shock wave cold works the surface of the workpiece and creates compressive residual stresses, which provide an increase in fatigue resistance properties of the part. A workpiece may be processed by producing a matrix of spots that cover all or part of the fatigue-critical zone of the part.
Laser shock processing is being used for many applications within gas turbine engines, such as leading and trailing edges of fan and compressor airfoils. These applications, as well as others, are in need of efficient quality assurance testing during production runs using laser shock processing. The quality of laser shock processing depends upon the quality of the laser beam, the plasma plume, and subsequently the resultant shock wave that produces the residual compressive stresses. It also depends on the coupling of the laser beam to the opaque overlay or surface.
For these reasons, during laser shock processing, several parameters of the laser beam, such as temporal profile, spatial profile, and beam energy are desirably measured for each laser shot. These parameters provide information about the quality of the laser beam and indirectly provide a potential metric of a plasma plume characteristic or the shock wave magnitude, i.e., the pressure generated by the plasma plume is a function of the characteristics of the laser beam. Furthermore, the pressure profile of the shock wave in the workpiece is a function of the plasma plume characteristics. However, the information about the quality of the laser beam does not provide direct information about the characteristics of the plasma plume or the shock wave.
Conventional methods of assessing the quality of laser shock processing such as high cycle fatigue testing of laser shock peened components provide sufficient measures, however, they are time consuming and costly. Furthermore, these methods are not acceptable for continuous production since it requires terminating the processing while the fatigue test results are acquired.
A method disclosed in U.S. Pat. No. 5,951,790, entitled xe2x80x9cMethod of Monitoring and Controlling Laser Shock Peening Using An In Plane Deflection Test Couponxe2x80x9d measures the quality of laser shock processing. However, this method requires multiple laser shots in the form of a pattern to generate sufficient in-plane deflection in the gauge. These multiple shots require time to set up and apply the pattern. The amount of in-plane deflection generated by this technique is relatively small, requiring an instrument to measure small deflections with a high degree of precision.
The present invention provides a fast, reliable, and efficient method of measuring the quality of laser shock processing using only a single laser shot in each test. According to one form of the present invention, each test is performed with a mechanical gauge that measures the impact produced by a plasma plume ignited by the laser pulse. The mechanical gauge can be formed in the shape of a strip, or a sheet, or any other shape that can provide for a measurable deflection in the direction substantially perpendicular to and away from the impacted surface.
According to one preferred embodiment of the invention, a gauge for ensuring the proper operation of a laser shock processing system comprises a test element, and a mounting means for holding the test element such that a portion of the test element is in the anticipated path of a laser pulse, wherein the test element sustains a deflection in the direction substantially perpendicular to and away from the impacted surface when subjected to a single laser pulse, the deflection being indicative of the magnitude of the shock wave. The test element is preferably formed in the shape of a strip, or in the shape of a sheet.
According to one form of the invention, the test element is coated with a first overlay and a second overlay, the first overlay being an opaque material, such as paint or tape. The second overlay is transparent, and typically comprised of water. The thickness of the second overlay has a significant effect on the amount of deflection in the test element when all other processing variables are held constant. The thicker the second overlay, the greater the amount of deflection. In this embodiment, the thickness of the second overlay is controlled. In the embodiment utilizing a strip, the mounting means of the invention comprises a means for gripping one end of the test element while positioning the second end of the test element in the anticipated path of the laser pulse. In the sheet embodiment of the invention, the mounting means comprises a base having a recess, wherein the test element is positioned over the recess and in the anticipated path of the laser pulse. A recess can be any portion of the base that does not contact the test element including, but not limited to, a hole, a notch, a groove, and the unsupported area below a test element that overhangs the base.
In an alternative embodiment of the invention, a method of measuring the quality of laser shock processing comprises the steps of mounting a test element in the anticipated path of the laser pulse, laser peening the test element with a single laser pulse, measuring the deflection of the test element in the direction substantially perpendicular to and away from the impacted surface, and comparing the deflection measurement to a previously generated measurement data set. The previously generated measurement data set includes at least one value selected from the group including test element dimensions, test element material composition, second overlay thickness, laser pulse duration, the laser-pulse rise time, test element deflection measurement, laser power density, laser pulse energy, imparted residual stress in a workpiece, fatigue strength of a workpiece, fatigue life of a workpiece, and shock wave magnitude.
In yet another embodiment of the invention, a method of measuring the quality of laser shock processing comprises the steps of generating a data set; mounting a test element in the anticipated path of the laser pulse; selecting the second overlay thickness; irradiating the test element with a laser pulse; measuring the deflection of the test element in the direction substantially perpendicular to and away from the impacted surface; and comparing the deflection measurement to the data set. In this embodiment of the invention, the data-set generating step includes the steps of selecting a test element with desired dimensions and material composition; mounting the test element in the anticipated path of the laser pulse; selecting the second overlay thickness; irradiating the test element with a laser pulse having a selected pulse duration; measuring the deflection of the test element; and correlating the deflection of the test element with the magnitude of the laser-generated shock wave or correlating the deflection of the test element with a material property of a workpiece, such as imparted residual stress, fatigue life, or fatigue strength. Correlation of the test element with shock wave and fatigue or other material properties is accomplished through separate measurements of shock-waves, material effects, or fatigue properties using substantially identical laser pulses. In most cases, it is necessary to use many laser pulses, applied to the workpiece in an overlapping pattern, to create the desired material or fatigue-property effect in the workpiece. For example, after laser shock peening the fatigue-critical area of a workpiece with overlapping spots of a selected laser pulse, the workpiece may be fatigue tested to determine the fatigue life of the workpiece. The measured fatigue life can then be correlated to a specific deflection of a test element using a substantially identical laser pulse. In this way, a single laser pulse can be used on a test element as part of a quality assurance procedure to ensure that the laser peening system will produce workpieces with the desired fatigue-life properties. According to the invention, the data set generating step is repeated until a sufficient number of test element dimensions, laser pulse metrics, and corresponding deflections are recorded and correlated to the desired properties of the workpieces, or more generically, to a desired residual stress profile within a selected material.
In a still further embodiment of the present invention, a method of selecting a test element for use in measuring the magnitude of a shock wave resulting from a single laser pulse comprises the steps of determining the properties of the laser system to be used; selecting the second overlay thickness; selecting the duration of the laser pulse; selecting the maximum amount of test element deflection desired; selecting the laser pulse impact position on the test element; and calculating the permissible composition and ranges of dimensions for the test element from the above values.
In yet another embodiment of the present invention, a method is disclosed of ensuring the proper operation of a laser shock peening system for a production laser-peening process comprising; laser peening a test element using a single laser pulse; measuring the deflection of said test element caused by said laser peening; determining if said deflection is within a predetermined range; and then laser peening production work pieces.
It is an advantage of the present invention that impulse monitoring can be accomplished with only a single laser pulse to a test element. Where previously, multiple laser pulses were required in the form of a pattern to generate sufficient in-plane deflection in a test element, the present invention allows for more efficient testing without compromising accuracy.
It is a further advantage of the present invention that quality control monitoring can be accomplished much more quickly and efficiently with the simple measurement of the deflection of the test element, and the subsequent comparison of that deflection to a pre-generated chart or data set.
In yet another embodiment of the present invention, a single test element is mounted in a fixture and secured in the anticipated path of two substantially-symmetrically opposed laser beams. The two substantially-symmetrically opposed laser beams, directed toward each other, are fired at substantially the same time wherein they may have substantially identical processing conditions and arrive at the processing plane at substantially the same time. The second overlay is applied to both sides of the gauge and the thickness of this overlay may or may not be the same thickness on both sides. In this case, the gauge will be used to ensure substantially similar processing conditions are produced for each surface of a two-sided laser peening operation. A measurable deflection in the gauge will indicate potentially non-symmetric processing conditions between the two laser beams or a potentially significant difference in the arrival time of the two laser beams on the target plane. For symmetric processing, substantially zero deflection on the gauge is desirable.
In some part-specific configurations, it may be desirable to produce a non-symmetric processing condition. If non-symmetric processing is desirable, the above-described method may be used, except that a measurable deflection is expected.
It is another advantage of the present invention that the shape, dimensions, and composition of a test element can be varied to accommodate a variety of materials, or varied applications.